xdev fixed

The issue reported here:
https://lists.freebsd.org/pipermail/freebsd-arm/2016-June/014065.html

> make XDEV=arm XDEV_ARCH=armv6 xdev
> 
> It failed!
> 
> sh /usr/src/tools/install.sh  -C -o root -g wheel -m 444  /usr/src/gnu/lib/libdialog/../../../contrib/dialog/dialog.h /usr/src/gnu/lib/libdialog/../../../contrib/dialog/dlg_colors.h /usr/src/gnu/lib/libdialog/dlg_config.h /usr/src/gnu/lib/libdialog/../../../contrib/dialog/dlg_keys.h //usr/armv6-freebsd/usr/include/
> ===> lib/libc++ (obj,all,install)
> /usr/obj/armv6-freebsd/usr/src/lib/libc++ created for /usr/src/lib/libc++
> echo libc++.so.1.full: //usr/armv6-freebsd/usr/lib/libcxxrt.a >> .depend
> c++ -isystem //usr/armv6-freebsd/usr/include -L//usr/armv6-freebsd/usr/lib  --sysroot=//usr/armv6-freebsd/ -B//usr/armv6-freebsd/usr/libexec  -B//usr/armv6-freebsd/usr/bin -B//usr/armv6-freebsd/usr/lib -mlong-calls -O -pipe -isystem /usr/src/lib/libc++/../../contrib/libc++/include -isystem /usr/src/lib/libc++/../../contrib/libcxxrt -nostdinc++ -nostdlib -DLIBCXXRT -MD -MF.depend.algorithm.o -MTalgorithm.o -Qunused-arguments  -std=c++11 -Wno-c++11-extensions  -c /usr/src/lib/libc++/../../contrib/libc++/src/algorithm.cpp -o algorithm.o
> In file included from /usr/src/lib/libc++/../../contrib/libc++/src/algorithm.cpp:10:
> In file included from /usr/src/lib/libc++/../../contrib/libc++/include/algorithm:624:
> In file included from /usr/src/lib/libc++/../../contrib/libc++/include/initializer_list:47:
> /usr/src/lib/libc++/../../contrib/libc++/include/cstddef:43:15: fatal error: 
>       'stddef.h' file not found
> #include_next <stddef.h>
>               ^
> 1 error generated.
> *** Error code 1


This should now be fixed in head as of r313907.  Sorry for the long
delay on it.  It likely came about due to the libc++ upgrade around
r300873 in May 2016.  I will MFC the fix to stable/11 once I get more
feedback or in 2 weeks otherwise.

-- 
Regards,
Bryan Drewery